String class

Catalog

What is? String class

frequently-used String Method

example

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What is? String class

String Class represents string ,Java All string literals in the program （ Such as "abc" ） Are implemented as examples of this class . Strings are constants ; Their values cannot be changed after creation .

frequently-used String Method

①str.length()   Gets the length of the string

②str.charAt()   Get a single character

③str.substring(1,2)  Intercepting string

④str.indexOf(" One ")    Get the index position of the specified character

example

Design a login program , Enter... Through the keyboard , The login name is “ Zhang San ”, The password for 123456, There are a total of three login opportunities .

public class Login {
    public static void main(String[] args) {
        String username = " Zhang San ";
        String password = "123456";
        Scanner scanner = new Scanner(System.in);
        int count = 0;
        for (int i = 0; i < 3; i++) {
            System.out.println(" Please enter a user name ：");
            String zhanghao = scanner.next();
            System.out.println(" Please input a password ：");
            String mima = scanner.next();
            if(zhanghao.equals(username)&&mima.equals(password)){
                System.out.println(" Login successful ！");
                break;
            }else{
                System.out.println(" Login failed ！ Please re-enter ！ You still have "+(2-i)+" Second chance ！");
                count++;
            }
        }
        if (count==3){
            System.out.println(" Too many failures , Please try again later ！");
        }
    }
}

analysis ： First of all, we can see that we need to use keyboard input , And there are a total of three login opportunities , If the user name and password are correct, there is no need to , So you can use a loop , When the number of times is known, you can think of using for loop , If correct, then break Out of the loop , meanwhile , You can use String Class equals Compare keyboard input with “ Zhang San ” and 123456 equally .

Enter a string on the keyboard , Traverse and output the string

public class StringTest {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.println(" Please enter the string ：");
        String a = scanner.next();
        for (int i = 0; i < a.length(); i++) {
            System.out.print(a.charAt(i));
        }
    }
}

analysis ： To traverse a string , We can think of taking a single character of a string , Export them , So you can use charAt(), Use the loop to output them one by one , The number of cycles is the length of the string , So you can think of it length.

Receive a phone number from the keyboard as a string , Block the middle four digits of the mobile phone number

public class StringTest02 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.println(" Please enter your mobile number ");
        String number = scanner.next();
        String before = number.substring(0,3);
        String after = number.substring(7);
        System.out.println(before+"****"+after);
    }
}

analysis ： Because it is necessary to block the middle four digits of the mobile phone number , So we can think of intercepting strings , Get the top three mobile phone numbers , The last four got , Intermediate use * Instead of just , So we need substring().